Ddelisle,
Assuming you want aesthetics and sound quality and you cannot go with free standing, then in-wall and in-ceiling are your next best choice. In terms of "issues" I would avoid intersections between the walls and ceiling as much as possible. If that is impossible, then some brands (like Revel) have boundary compensation switches in their speakers. You also want to think about where you want the audio to go. Most ceiling speakers point straight down. Others offer a 15 degree angle so that you can direct the tweeter. Some of these directional speakers specifically state that they can be used as the front two channels of an audiophile system or three together to form the front three of a home theater.
Thus, that goes to the next point besides boundaries, etc. and that is how deep the speakers go. You want to carefully look at the crossover point of the speakers and figure out what (if anything) you will do with a sub. If you will do a sub, will it be free-standing or in-wall? Some speakers will go down into the 40hz range. So, I would tend to be more particular about adding some extra insulation. When I did my second, smaller theater/tv area with all in-walls and in-ceiling we used heavier (3/4") sheetrock and soundboard. That has created an incredible dampened wall. My ceiling is an open cavity with insulation and is less of an issue. If you do decide to go with an in-wall or in-ceiling sub, you need to be very careful about the sheetrock used in the wall for vibrations. Some companies (like Paradigm and others) offer in-wall subs that don't vibrate.
However, and this goes to the next point, if you ever think that there is even the most remote chance of the ceiling getting water, then tie the ceiling speakers with a small piece of wire to a nearby joint or hook that you install. Thus, even if the ceiling gets wet, you don't want the speaker falling down and potentially onto someone. I personally did this with all my ceiling speakers as a precaution.
Additionally, if you are concerned about the insulation in the cavity, you can likewise purchase in-ceiling and even in-wall speakers that have their own enclosures so that you don't need to worry about the acoustics of the cavity.
And finally, with ceiling speakers, because you cannot fool around with placement, you may want to have a receiver with room correction and multi-zones. If you are only using the speakers for background, then I wouldn't fool around with any room correction.
Those are some of the "big" issues that you should think about.
For ceiling speakers, I suggest the Revel C760L as being the ideal with both boundary compensation, an angled tweeter design so that you can use with home theater and two-channel, and also a closed back for accurate voicing. You can see it here: http://revelspeakers.com/Products/Details/69. This is a great speaker and I have personal experience with the model this one replaced. The Revels have magnetic grilles and square or round covers.
This particular model only goes down to 80hz and ideally you want to mate it with a subwoofer.
I also recommend Atlantic Technology speakers. They too offer closed back design. I personally prefer the Revels to the Atlantic Technology products if it is a more audio-critical installation. I find the Revels to have a much cleaner midrange. If you are looking for other "audiophile" architectural speakers, there are certainly many to choose from as the previous posts have indicated. Paradigm and B&W likewise offer architectural speakers. Paradigm especially has put some heavy emphasis with architectural subs.
